The present invention relates to a piezoelectric transformer, and more particular to the piezoelectric transformer for a fluorescent lamp including a piezoelectric block having smaller electrodes at the center region of each side than that at corner regions to minimize the stress at the center region of the piezoelectric block in order to use for general fluorescent lamp.
Since BaTiO3 has been introduced at the mid-1940s as a piezoelectric ceramic material, ceramic materials having good piezoelectric characteristics, such as Pb(Zr,Ti)O3 (PZT), have been found to be useful for electrical devices. The PZT which is the solid solution of PbZrO3 and PbTiO3 is a 2-element based perovskite structure having good piezoelectric characteristics. Recently, 3-element based perovskite has been studied to vary easily the composition of the 2-element based perovskite and to improve the piezoelectric characteristics. For example, Pb(Mg,Nb)O3xe2x80x94Pb(Zr,Ti)O3, Pb(Mg,Ta)O3xe2x80x94Pb(Zr,Ti)O3, and Pb(Mn,Nb)O3xe2x80x94Pb(Zr,Ti)O3 are developed as 3-element based complex perovskite compounds.
FIGS. 1A-1B show an applicable example of the piezoelectric material, i.e., a band pass filter for the amplitude modulation. FIG. 1A is a plan view of the band pass filter and FIG. 1B is a sectional view along the line Axe2x80x94A of FIG. 1A. As shown therein, the band pass filter comprises a piezoelectric block having a hexahedron shape and having electrodes on the upper and lower faces thereof. The electrodes on the upper face are input/output electrodes 3 and 5 and the electrode on lower face is a common electrode 7.
When the AC voltage corresponding to the resonant frequency is supplied to the input electrode 3, the electrical signal is converted into a strong mechanical vibration near the input electrode and the converted vibration is transmitted to the output electrode. As a result, a voltage proportional to the resonant frequency is outputted from the output electrode 5.
Piezoelectric materials have been introduced at the end of the 1910s and applied to various electronic devices such as a high voltage generator, an ultrasonic generator, sound facilities, a 45.5 KHz IF filter for AM radio and a 10.7 MHz filter for FM radio, communication devices, and various sensors. Further, this piezoelectric material has been used for a resonator or filter for communication devices, and an inverter of a cold cathode tube for a backlight of a liquid crystal display. In addition, a piezoelectric transformer used for an inverter has also been introduced.
The structure of a typical transformer composed of a piezoelectric material is shown in FIGS. 2A-2B. FIG. 2A is a perspective view and FIG. 2B is a sectional view. This transformer is Rosen type transformer having vibrations modes in the thickness and length directions. As shown therein, the transformer is polarized in the thickness direction at an input electrode region on a part of the upper and lower faces. In an output electrode region, further, it is polarized in the length direction. When the AC voltage corresponding to the resonant frequency is supplied to the input electrodes 3a and 3b to raise the voltage of the piezoelectric transformer, the supplied electrical signal is converted into a strong mechanical vibration of the length direction near the input electrodes and then transmitted to the output electrode to generate the longitudinal vibration. Then, the longitudinal mechanical vibration is outputted from the output electrode 5 as an increased high voltage having a high frequency that is identified with the frequency of the voltage supplied to the input electrode.
The increase of the voltage at the output electrode is maximized when the frequency of the input voltage is identified with the mechanical vibration frequency at the output electrode. Further, the voltage-increase ratio of the piezoelectric transformer, which is dependant upon the load impedance of the material, is maximized when a relatively high load impedance is applied to the output electrode. In case of lower load impedances, the voltage increase ratio is less than several tens times.
When the piezoelectric transformer is used for a lamp such as a cold cathode tube and a fluorescent lamp, it has a different load impedance in accordance with the kind of lamp. However, if the piezoelectric transformer is made under optimum fabrication conditions, the high voltage-increase ratio can be maintained under the condition of a lower load impedance. In the normal state that the impedance is decreased after lighting, further, the voltage-increase ratio can suitably be maintained so that the piezoelectric transformer can be used for a lamp such as a cold cathode tube and a fluorescent lamp.
Recently, a piezoelectric transformer having an outline vibration mode shown in FIGS. 3A-3B has also been introduced. As shown in FIGS. 3A-3B, a piezoelectric transformer of an outline vibration mode has a structure similar to that of the band pass filter shown in FIGS. 1A-1B except for the shape of the electrodes. That is, the electrode in FIG. 3A has a circular shape, rather than a rectangular shape as in FIG. 1A. In this piezoelectric transformer, the output electrode 5 is disposed on the upper face of the piezoelectric block 1 at a predetermined distance from the input electrode 3 and the common electrode 7 is disposed on the lower face of the piezoelectric block 1.
When a voltage is supplied to the piezoelectric block 1 through the input electrode 3, the electrical signal is converted into a mechanical vibration directed to a side portion from the central portion of the piezoelectric block 1 and then the signal proportional to the mechanical vibration is outputted though the output electrode 5. This piezoelectric transformer may be used for a liquid crystal display of a notebook computer and a low-consumption transformer.
There, however, is a problem in that piezoelectric materials are not applicable to high power transformers. Since the Rosen or the deposited Rosen type piezoelectric transformer (U.S. Pat. No. 6,037,706) has a complex structure and outputs a low output signal, it is not applicable to a fluorescent lamp. In the outline vibration mode-piezoelectric transformer, the deposited structure of the piezoelectric material should be studied because of the problem of the material. The electrodes of the circular and rectangular shapes in the outline vibration mode-piezoelectric transformer are disclosed in xe2x80x98Design of Fluorescent lamp with PFC using a power piezoelectric transformer, Sung Jin Choi, IEEE(1998. 2. 15), P1141xe2x80x99. In this transformer, however, there is a problem that the output signal is low.
Further, there is a problem in that the stress is maximized in a part of the piezoelectric block so that the piezoelectric device may be destroyed and the efficiency thereof may be deteriorated, when the electrical signal is converted into the mechanical vibration.
It is an object of the present invention to provide a piezoelectric transformer in which the electrode at the low vibration region of the piezoelectric block, that is, the central region of each side is minimized to eliminate the heat caused by the stress and prevent the damage of the piezoelectric block.
In order to achieve the object, in the piezoelectric transformer of the present invention, the size of the electrode at the stress generation region on the upper face of the piezoelectric block is decreased to minimize the heat caused by the stress. The stress is chiefly generated near the central region of each side of the piezoelectric block. By minimizing the size of the electrode at this region, thus, the stress is also minimized and as a result, this piezoelectric transformer is applicable to high power transformers. For the minimized electrode, the electrode can be formed in the diamond shape or a cross shape. The shape of the electrode can be formed in various shapes, without limitation.